Anal. Chem. 1996, 68, 1245-1249
Embossable Grating Couplers for Planar Waveguide Optical Sensors Brigitte L. Ramos and Steven J. Choquette*
Analytical Chemistry Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Nicholas F. Fell, Jr.
Ignition and Combustion Branch, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005
Planar optical waveguides are an attractive tool for use in analytical chemistry and spectroscopy. Although similar to fiber optics, planar waveguides have been slow to be commercially accepted due to the difficulty of coupling light into the guide. Generally, prism coupling is the method of choice in the laboratory, as efficiencies approaching 80% can be reached. However, prisms are impractical for routine use for several reasons: expensive positioning equipment is required, coupled power is sensitive to environmental fluctuations, and prism coupling prohibits the fabrication of a truly planar device. The use of thin gratings on the surface of the waveguide allows for a two-dimensional structure to be maintained, while providing enough efficiency to be useful as a sensor. Our research efforts focus on developing a technique to make inexpensive, reproducible gratings that are easy to fabricate. By chemically modifying the surface of a commercial grating with a suitable release agent, it is possible to emboss replica gratings onto a variety of waveguide types. The fabrication of embossed gratings will be described, and their performance on glass, ion-diffused, polymer, and semiconductor waveguides will be presented. Planar optical waveguides are an attractive tool for use in analytical chemistry and spectroscopy. A wide variety of inorganic and organic materials have been used to fabricate thin-film waveguides, and as a result, planar guides can be engineered for specific chemical applications. As the evanescent wave is easily accessed, a number of papers have addressed the use of planar waveguides for bio/chemical sensors. Attenuation,1-4 fluorescence,5,6 and interferometric7 sensors have been reported, as has the use of waveguides for enhanced Raman8,9 spectroscopy. Unlike fiber optics, planar waveguides have been slow to be widely accepted due to the difficulty of coupling light into the waveguide. In the laboratory, prism coupling1,10 is the predomi(1) Saavedra, S. S.; Reichert, W. M. Appl. Spectrosc. 1990, 44, 1210-1217. (2) Stephens, D. A.; Bohn, P. W. Anal. Chem. 1989, 61, 386-390. (3) Choquette, S. J. Ph.D. Thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA, 1988. (4) Kuhn, K. J.; Burgess, L. W. Anal. Chem. 1993, 65, 1390-1398. (5) Choquette, S. J.; Locascio-Brown, L.; Durst, R. A. Anal. Chem. 1992, 64, 55-60. (6) Fell, N. F.; Bohn, P. W. Anal. Chem. 1993, 65, 3382-3388. (7) Heideman, R. G.; Kooyman, R. P. H.; Greve, J.; Altenburg, B. S. F. Appl. Opt. 1991, 30, 1474-1479. (8) Levy, Y.; Imbert, C.; Cipriani, J.; Racine, S.; Dupeyrat, R. Opt. Commun. 1974, 11, 66-69. (9) Rabolt, J. F.; Santo, R.; Swalen, J. D. Appl. Spectrosc. 1979, 33, 549-551. This article not subject to U.S. Copyright. Published 1996 Am. Chem. Soc.
nant method, followed by endfire and grating coupling.11 Prism coupling, which operates on the principle of frustrated total internal reflectance, and endfire coupling, which uses fiber optics or a lens to introduce light directly into the polished endface of the waveguide, are highly efficient methods, as typically 80% of the laser beam is coupled into the waveguide. The use of prisms and fibers does not damage the waveguide, and the various elements (prisms, fibers, and lenses) are reusable. They are impractical for routine use, however, as both coupling methods require expensive positioning equipment. Prism coupling is sensitive to environmental fluctuations and destroys the twodimensional geometry of the planar waveguide. Diffraction or reflection gratings for light coupling into planar waveguides are more practical than prisms or fibers for routine use. Although the coupling efficiency observed with gratings is reduced, the twodimensional nature of the guide is conserved, and gratings are generally more robust than prisms. Furthermore, the coupled power is immune to environmental fluctuations because the grating is often embedded in the waveguide. Grating couplers are commonly fabricated using techniques based on holography. This approach involves an exposure step using a single mirror which creates an interference pattern between two spatial halves of a laser beam.12,13 The exposed photoresist acts as a mask for chemical etching of the underlying waveguide or substrate to form a periodic grating structure. This process can be time consuming, since this method involves an exposure followed by a chemical etch, and designing blazed gratings requires additional fabrication steps. The use of an embossing technique, where the surface relief pattern of a master grating is pressed into a suitable material, may provide a fast and economical method to form grating couplers for routine use. Methods which include peeling a polymer grating from a master and transferring it to a waveguide,14 or directly embossing a grating into hard inorganic sol-gel waveguides, have been reported.15,16 Previous methods that employ a heat treatment after embossing are often plagued with problems of shrinkage and cracking that affect the grating profile, depth, and structure. It (10) Tien, P. K.; Ulrich, R.; Martin, R. J. Appl. Phys. Lett. 1969, 14, 291-294. (11) Tamir, T. In Integrated Optics; Tamir, T., Ed.; Springer-Verlag: New York, 1979; pp 84-134. (12) Mai, X.; Moshrefzadeh, R.; Gibson, U. J.; Stegeman, G. I.; Seaton, C. T. Appl. Opt. 1985, 24, 3155-3161. (13) Malag, A. Opt. Commun. 1980, 32, 54-58. (14) Wei, J. S.; Tan, C. C. Appl. Opt. 1976, 15, 289. (15) Lukosz, W.; Tiefenthaler, K. Opt. Lett. 1983, 8, 537-539. (16) Roncone, R. L.; Weller-Brophy, L. A.; Weisenbach, L.; Zelinski, B. J. J. J. Non-Cryst. Solids 1991, 128, 111-117.
Analytical Chemistry, Vol. 68, No. 7, April 1, 1996 1245
has been shown that embossed patterns can exhibit up to 60%70% shrinkage after a postemboss bake.16 Another embossing technique involves coating a metal release agent onto a master grating;17 however, it is necessary to recoat the master grating each time a copy is made. We present a modification of previously reported methods which alleviates the need to thermally process the embossed gratings or to recoat the master grating with each use. By chemically modifying the surface of a commercial master grating with a suitable release agent, it is possible to emboss replica gratings onto a variety of waveguide types. Many replicates can be produced from a single master, thereby facilitating the fabrication of inexpensive, reproducible gratings which yield sufficient efficiency to be used as waveguide couplers. Furthermore, this method is applicable to the most common types of planar guides. The use of embossed gratings with float glass, ion-diffused, polymer, and semiconductor waveguides will be discussed. EXPERIMENTAL SECTION Reagents and Materials. UV curable epoxy, No. 81, was obtained from Norland Products Inc. (New Brunswick, NJ). NOA 81 was chosen for its rapid curing rate, high transparency between 400 and 3000 nm, and refractive index of 1.56. Reflection gratings (1200 line/mm, 17° 27′ blaze angle) and glass prisms (Schott glass SF-2, nd ) 1.644) were purchased from Edmund Scientific (Barrington NJ). Cubic zirconia prisms (Lot No. PRE 0302, nd ) 2.158) were purchased from Precomp (Great Neck, NY). Microscope slides were obtained from Erie Scientific (Portsmouth, NH) and Kimble (Toledo, OH). Polystyrene (MW ) 280 000, lot No. 00320EF) was purchased from Aldrich (Milwaukee, WI). (Tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane was from Petrarch Systems (Bristol, PA). Sodium nitrate, silver nitrate, ammonium hydroxide, and hydrogen peroxide were all of reagent grade. All materials were used as received. Instrumentation. The UV source (365 nm) used to cure the epoxy gratings was from Spectroline, Model enf-260c, 115 V, 60 Hz, 0.20 A (Spectronics Corp., Westbury, NY). Waveguide characterization was performed on a Metricon Model 2010 Prism Coupler (Metricon Corp., Pennington, NJ). Measurements obtained in the laboratory employed a 5 mW, linearly polarized HeNe laser (Uniphase, Monteca, CA) and a photodiode detector (PIN10D type, Model 818-SL, Newport Corp., Irvine, CA) with a transimpedance amplifier (Model 101C, United Detector Technology, Hawthorne, CA). Interference measurements for polystyrene waveguides were obtained with a UV-vis 260 spectrophotometer (Shimadzu, Kyoto, Japan). The spin-coater was constructed inhouse using a variable speed HST110 motor controller (G. K. Heller Corp., Floral Park, NY). Preparation of Waveguides. Green float glass waveguides were obtained as microscope slides from Erie Scientific. Glasses fabricated by the float process commonly have tin oxides incorporated into the surface. The presence of the oxide layer increases the refractive index at the surface of the glass substrate, resulting in a layer that supports one or more guided modes.18 Silver ion-diffused waveguides were fabricated by immersing clean microscope slides (Kimble) in a 0.25 wt % AgNO3/NaNO3 solution at 320 °C for 15 min. The guides were removed, cooled, and (17) Christensen, D.; Dyer, S.; Herron, J.; Hlady, V. Proc. SPIE-Int. Soc. Opt. Eng. 1992, 1796, 20-25. (18) Osterberg, H.; Smith, L. W. J. Opt. Soc. Am. 1964, 54, 1078-1084.
1246 Analytical Chemistry, Vol. 68, No. 7, April 1, 1996
Figure 1. Schematic of grating orientation on (A) float glass and ion-diffused waveguides, (B) polystyrene waveguides, and (C) Si3N4 waveguides.
thoroughly rinsed in deionized H2O. Any precipitated (reduced) silver was removed by wiping the surface with HNO3. Polystyrene waveguides were fabricated by spin-coating. A clean microscope slide was flooded with a 50 mg/g solution of polystyrene in toluene that was previously filtered with a 0.2 µm stainless steel frit. Excess polymer solution was spun off, and the film was formed by spinning at 2000 rpm for 1 min. The resulting waveguides were allowed to dry in the presence of toluene vapors for 1 day before use. Silicon nitride waveguides were fabricated in-house using standard silicon foundry procedures. A 1.8 µm thick SiO2 buffer layer was grown on a 3 in. p-type silicon wafer with a 〈111〉 orientation. Dichlorosilane was reacted with a stoichiometric excess of ammonia in a low-pressure chemical vapor deposition (LPCVD) process at 750 °C to form silicon nitride layers between 1200 and 3400 Å thick on top of the oxidized buffer layer. These wafers were then annealed at 1000 °C for 60 min in flowing nitrogen. Following the first annealing step, a 1400 Å layer of low-temperature oxide (SiO2) was deposited on the Si3N4 waveguide. The final annealing step was performed at 1000 °C for 45 min in flowing nitrogen. Fabrication of Embossed Gratings. Ion-Diffused Waveguides. Commercial gratings were silanized from vapor phase with a 1% (v/v) solution of (tridecafluoro-1,1,2,2,-tetrahydrooctyl)1,1-trichlorosilane in toluene at 50 °C for 35 min. The gratings were allowed to dry at room temperature for several hours. Embossed gratings were fabricated by placing a small drop of UV curable epoxy onto the waveguide surface and pressing the epoxy drop onto the silanized master grating. For thin (0.1-2 mm) glass substrates, it is possible to cure the epoxy through the substrate with a UV lamp. After a 1 min exposure, the glass substrate is pulled away from the release-coated master, leaving an embossed grating on the waveguide surface. This procedure was used to fabricated gratings on float glass and ion-diffused waveguides (Figure 1A). To preserve the commercial (master) gratings, all waveguide gratings were embossed from copies of the master grating known as “submaster” (SM) gratings. The submaster was formed by first embossing a large (12 mm diameter) epoxy grating onto a glass substrate, which was subsequently aluminized (∼100 nm)
Table 1. Index, Thickness, and Waveguide Efficiency of Thin-Film Planar Waveguides substrate index, Ns
mode
Neff
film thickness, Tf
film index, Nf
waveguide efficiency (%)a
float glass (n ) 10) Ag+ diffuse (n ) 4)
1.5168 ( 0.0001 1.5163 ( 0.0003
(SIMS) 3.7660c 3.1070 2.3300 1.3840 1.656 (ui )0.056)d
18.82 (ui ) 2.48) 0.64 (ui ) 0.40)
1.5163 ( 0.0003
1.5916e
5.97 (ui ) 9.95)
Si3N4 (n ) 2)
1.4579 ( 0.0001
1.5184 (ui ) 0.0002) 1.5198 (ui ) 0.0008) 1.5333 (ui ) 0.0009) 1.5487 (ui ) 0.0021) 1.5651 (ui ) 0.0020) 1.5195 (ui ) 0.0030) 1.5651 (ui ) 0.0019) 1.5394 (ui ) 0.0003) 1.8954 (ui ) 0.0001)
nab nab
poly(styrene) (n ) 4)
0 3 2 1 0 1 0 1 0
0.3285 (ui ) 0.0001)
2.0146 (ui ) 0.005)
8.39 (ui ) 1.34)
waveguide
a n ) 3 for efficiency measurements. b Not applicable. c Calculated turning point for each mode based on inverse WKB method. d Determined using inteference method. e nD20 value reported by supplier.
by vacuum deposition. The aluminized grating was then silanized with the trichlorosilane release reagent. Embossed gratings were then fabricated as described above. Gratings prepared from the SM exhibited diffraction efficiencies identical to those exhibited by gratings prepared directly from the commercial master. Polymer Waveguides. Gratings were prepared by evaporating a thin (∼100 nm) coat of aluminum on a SM grating. The coated SM was then pressed onto a small drop (
